Field of the Invention
The invention relates to a method for coating the surface of a substrate, such as inorganic particles, with a metal oxide, wherein a reduced mass-related specific surface area of the metal oxide coating is achieved. In particular, the invention relates to the surface coating of titanium dioxide pigment particles with aluminum oxide.
Technological Background of the Invention
Inorganic particles, particularly inorganic pigment particles, are often surface-coated in order to modify certain properties, such as abrasion resistance, surface charge, dispersing properties, as well as acid resistance and light stability. Thus, U.S. Pat. No. 2,885,366 describes the application of a dense silicon dioxide coat to substrate particles, such as nickel or iron powder, glass fibers or titanium dioxide. EP 0 130 272 A1 and U.S. Re 27,818 disclose the coating of color and white pigments with different oxides and hydroxides in order to adjust properties of the thus obtained pigments.
Especially titanium dioxide pigments are also coated with hardly soluble aluminum compounds, mainly with anhydrous and/or aqueous aluminum oxide compounds, in the course of their production method. In this way, the compatibility of the pigments with the remaining components in the user systems, such as in coatings, is increased, i.e., typical properties of the white pigment, such as dispersibility, brightening capacity or opacity, are improved.
The skilled person knows different methods for precipitating aluminum oxide/hydroxide from the patent literature. U.S. Pat. No. 6,616,746 B2 relates to a method for precipitating aluminum oxide at a moving pH, in which an aqueous titanium dioxide suspension of alkaline pH is employed, then sodium aluminate is charged first, and subsequently sulfuric acid is added until a pH of 5 is reached.
U.S. Re 27,818 discloses a similar procedure in which aluminum sulfate is charged first. Subsequently, a neutral pH is set with a base or another alkaline reacting substance, and aluminum oxide is precipitated in parallel.
However, aluminum oxide is usually precipitated at a constant pH. Thus, a water-soluble alkaline aluminum compound and an acidic compound, such as an acid or a water-soluble acidic aluminum compound, are added to the suspension at a constant pH, and aluminum oxide is precipitated. Alternatively, there may also be employed a water-soluble acidic aluminum compound and an alkaline compound, such as a base, or a water-soluble alkaline aluminum compound to precipitate the aluminum oxide. These method variants are disclosed, for example, in EP 1 603 978 B1 and EP 1 989 264 B 1.
However, the known methods for coating the surface of titanium dioxide pigments with aluminum oxide/hydroxide lead to further modifications of the particle surface, for example, an increase of the mass-related specific surface area, which can be determined by BET, or a shift of the isoelectric point. In the field of coatings, the additional surface consequently leads to an increased demand for additives, such as dispersants and binders. This is disadvantageous due the additional cost for the formulation and the limitation of the degrees of freedom for optimizing the formulation.
Therefore, there is a need in the art for a method by which a desired amount of aluminum oxide can be applied, and nevertheless properties such as the specific surface area and the position of the isoelectric point can be controlled more independently. Further, there is a need for coating systems with a lower proportion of dispersant and binder.